AW Ursae Majoris

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

AW Ursae Majoris: a close, eclipsing binary in Ursa Major

AW Ursae Majoris (AW UMa) is a pair of stars so close they share a common outer region. It’s an A-type W Ursae Majoris variable, meaning its brightness changes as the two stars eclipse each other every orbit.

Brightness and visibility
- Visible light magnitude: about 6.83 at maximum light; it dips to 7.13 during the primary eclipse and to 7.08 during the secondary eclipse.
- Distance: roughly 221 light-years from Earth.
- It moves across the sky fairly quickly for a star system, and it is slowly drifting toward us at about 17 km/s.

Orbit and orientation
- Orbital period: about 0.439 days (roughly 10.5 hours).
- The two stars orbit very close to each other in a nearly circular path.
- The orbit is tilted relative to our line of sight by about 78 degrees.

What the two stars are like
- Primary star: about 1.8 times the mass of the Sun, about 1.5 times the Sun’s radius, and a surface temperature near 7,000 K.
- Secondary star: much smaller, about 0.14 solar masses, with a radius likely less than about 0.7 solar radii, and a surface temperature in the range of about 6,200–6,900 K.
- The mass ratio is very low: the secondary has only about 8% of the primary’s mass.
- The system is thought to be a contact binary, where the stars share a common outer layer or belt, rather than being a clean, separate pair.

What this means for the system’s history
- The two stars have exchanged a lot of mass. Models suggest the current secondary was originally the more massive star and transferred much of its mass to the companion, changing their appearances and how they glow.

Possible companions
- Some research hints at the presence of a third body with about the mass of the Sun on a roughly year-long orbit, which could influence the system’s behavior.
- There is also a nearby star far enough away that it could be a distant, physically associated companion.

Overall, AW UMa is a well-studied, nearby example of a very close, mass-transfer binary system that helps astronomers understand how such extreme stellar pairs form and evolve.